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POHow does this recursive code to find all permutations of a string work?
 

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  1. POHow does this recursive code to find all permutations of a string work?
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    <p>I am doing <a href="http://projecteuler.net" rel="nofollow">Project Euler</a> problems. I am currently working on the circular primes problem</p> <blockquote> <p>The number, 197, is called a circular prime because all rotations of the digits: 197, 971, and 719, are themselves prime.</p> <p>There are thirteen such primes below 100: 2, 3, 5, 7, 11, 13, 17, 31, 37, 71, 73, 79, and 97.</p> <p>How many circular primes are there below one million?</p> </blockquote> <p>Although checking if something is primes was easy for me, I could not figure out how to get all the permutations of the numbers. After a good bit of searching for tips on an algorithm to do that, I came across a website which gave code for that in Java, which I adapted to PHP below. However, before proceeding with the problem, I would really like to understand what exactly the different bits of the code are doing, especially in the for loop. What I understand of it so far is that in the for loop, it is starting with an empty prefix and then looping through the string and adding a single element from the string to the prefix, until there is only one element left in the original string, at which point, it echoes it out. Am I understanding this correctly? If not, what am I missing?</p> <pre><code>&lt;?php getallcombos("","1234"); function getallcombos($prefix,$string){ if(strlen($string)==1){ echo $prefix.$string."&lt;br&gt;"; } $array=str_split($string); for($i=0;$i&lt;strlen($string);$i++){ $newstr=substr($string,0,$i).substr($string,$i+1); getallcombos($prefix.$array[$i],$newstr); } } ?&gt; </code></pre>
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      1. POHow does this recursive code to find all permutations of a string work?
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    1. COCheck my answer for this [here](http://stackoverflow.com/questions/19226649/how-does-the-algorithm-for-recursively-printing-permutations-of-an-array-work-ex)
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      1. POHow does this recursive code to find all permutations of a string work?
      1. USAlma Do
    2. COOr mine, here: http://stackoverflow.com/questions/19226649/how-does-the-algorithm-for-recursively-printing-permutations-of-an-array-work-ex/19227988#19227988 :) (It has a jsFiddle.)
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      1. POHow does this recursive code to find all permutations of a string work?
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    3. CODo you want *rotations* of the digits? Or *permutations* of the digits? That is, 197 has three rotations: 197, 719, 971. But there are 6 permutations: 179, 197, 719, 791, 917, 971.
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      1. POHow does this recursive code to find all permutations of a string work?
      1. USJim Mischel
 

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